It is known from experience with primary and secondary oil recovery methods that the efficacy of the known oil displacement processes is limited, because the fluid injected to displace and drive out the oil does not flood the entire volume of the oil reservoir.
It was concluded that, while on a microscopic scale a displacement coefficient of close to 1, can be achieved, the oil yield is still not satisfactory because the coefficient of volume flooding i.e. the size of the flooded reservoir volume is too small.
This is so because:
(a) the fluid used for displacement and for driving the oil out cannot flood but a part of a the oil bearing rock formations due to its micro- and macroscopic heterogeneity; and
(b) parts of the reservoir with considerable oil content remain untouched due to the difference in mobility between the oil being displaced and of the displacing fluid.
This undesirable occurrence in the reservoir is also known as "fingering" or "disjoining".
The term "vertically inhomogenous" as used throughout the specification and the claims, means that the permeability of the strata in the vertical direction is by at least an order of magnitude lower than the permeability in the horizontal direction. This occurs, for example, when parts of the reservoir are separated from each other by at least one or more marl stratum.
The term "secondary recovery" as used throughout the specification and the claims is intended to refer to any level of recovery that is subsidiary to primary recovery.
Another problem that occurs in secondary recovery processes takes place when working with a drive-out gas which will mix, partially or not at all with the oil and gas. In such cases insufficient flooding of the reservoir volume may occur. The large difference between the viscosities of the gas and the oil results in a pronounced "finger formation" and, on the other hand, the gas injected into heterogeneous reservoirs passes quickly through parts having good permeability and reaches the production well too fast without providing a suitable yield of oil.
Various other processes were suggested in the prior art to avoid the aforementioned results. A general feature of such other processes is that the displacing fluid is forced towards low-permeability parts of the reservoir by decreasing the permeability of those parts which the fluid penetrates easily.
Injecting gas and water alternatively or simultaneously is a method that is generally applied for increasing the efficiency of volume flooding [Caudle, B. H., Dyes, A. B., AIME 1958, 213-81]. Gas permeability of the stratum decreases in the three-phase flooding zone that is formed in this manner, resulting in some reservoirs in an increased efficiency of volume flooding.
According to U.S. Pat. No. 3,096,821 an increased efficiency of volume flooding is achieved by simultaneously injecting water and gas that mixes with oil into the reservoir.
U.S. Pat. No. 3,599,715 relates to a process wherein the mobility of the displacing gas is decreased by adding foaming agents to the gas.
U.S. Pat. No. 3,342,256 describes a process wherein the efficiency of oil production is increased by using carbon dioxide for forming a foam in situ in the reservoir.
A combined process for crude oil production is described in Hungarian Patent No. 177,703,wherein efficiency of gas flooding was improved by alternately injecting foaming agents and a disperse system into the reservoir.
U.S. Pat. No. 3,866,680 describes a process wherein the undesired effect of heterogeneity can be eliminated by injecting an emulsion.
The fast "break-through" of a miscible fluid, according to U.S. Pat. No. 4,129,182 can be avoided by first applying a solution such as a polymer solution, to decrease the permeability.
Hungarian Patent No. 173,636 describes a process, wherein the efficiency of an underground burning process is increased by injecting in metal additives and water.
It is known from U.S. Pat. No. 2,903,065 and USSR Patent No. 1,030,534 that efficiency of water flooding can be increased by injecting a salt solution having a higher temperature than the reservoir, and salt-crystals precipitate due to cooling.
None of the known gas driving displacement techniques were found to offer a satisfactory solution. A further disadvantage of these prior art processes is that their application requires expensive foaming agents such as detergents, polymers, dispersing agents and, therefore, these processes are very costly and often uneconomical.
Use of the known processes is also hindered by the fact that oil-bearing rock and the fluid in the stratum may change the composition of the injected displacing compounds, resulting in a flooding process that can get out of control.
The aim of the present invention is to provide a process for increasing the efficiency of volume flooding of oil reservoirs exploited by a gas injection procedure.